RegionMask

I'm trying to update an old script for creating hemispheres with WorldEdit.

The script uses a RegionMask. The RegionMask is made over the top half of the sphere.
From my understanding, a RegionMask should only allow modification within the region.
Everything is fine, except the mask doesn't allow anything to be placed.
When I check if a Vector is within the RegionMask, it says true, but no blocks are placed, even in the Vector I checked.
When I remove sess.setMask(regionMask), the sphere is successfully created, but this is not what I want.